Understanding the mechanisms of immune defense against pathogens is important for preventing and treating human infectious diseases. The importance of the innate branch of the human immune response is increasingly recognized as crucial in defense against many pathogens. Innate defense mechanisms of invertebrates and vertebrates show striking similarities and are evolutionary conserved. The fruitfly Drosophila melanogaster has provided an unbiased and powerful tool for examination of innate response against many types of microbial pathogens. However, it has not yet been used to explore innate response against bacteria in the Class Mollicutes, which includes important pathogens of humans such as Mycoplasma pneumoniae. The major goal of this proposal is to develop Drosophila as a model system for studying innate immunity against mollicutes, taking advantage of naturally infecting mollicutes of Drosophila (i.e., genus Spiroplasma). In the proposed study, the hypothesis is that mollicutes that naturally infect Drosophila elicit an innate immune response in the fly, which varies depending on the phenotypic effects (from mild to severe) of the bacterium strain on the host. These innate responses are likely to be conserved across invertebrates and vertebrates;thus, this work will serve as the foundation to study whether Drosophila innate immune response can help develop treatments for mollicute-related illnesses in humans and other vertebrates. The aim of the proposed project is to identify factors involved in the interaction between Drosophila and Spiroplasma via the following Specific Aims: Aim I. Establish whether the presence of Spiroplasma in Drosophila induces changes in the hemocyte transcriptome via hemocyte microarray experiments. Aim II. Establish whether the presence of Spiroplasma in Drosophila induces changes in the hemolymph proteome via proteomics/peptidomics experiments. PUBLIC HEALTH RELEVANCE The proposed research will use the fruitfly Drosophila melanogaster to understand the innate response of organisms against bacteria from the class Mollicutes. Bacteria in the class Mollicutes include important pathogens of humans such as Mycoplasma pneumoniae. By understanding how Drosophila, which lacks an adaptive immune system, interacts with Mollicutes, we may be able to understand the contribution of the human innate response to defense against Mollicutes.